The adjustment of the sails on a modern sailboat having a mainsail and overlapping jib or genoa foresail to obtain maximum forward hull speed for the boat involves taking into consideration a large number of interrelated variables, many of which are constantly subject to change as the sailboat is underway. The selection of sail settings for maximum hull speed when the sailboat is on a windward tack is a matter requiring a high degree of skill from the helmsman and crew particularly if maximum performance on course is required as in competitive racing where the windward leg of the contest is generally conceded to be the most critical phase of the competition.
Generally, the helmsman relies primarily upon his knowledge and experience to adjust the mainsail and jib in relation to the heading of the craft with respect to the prevailing wind. As a quantitative indication various forms of relative wind or through the water hull speed indicators may be employed. One of the difficulties of the helmsman's self-evaluation of his performance from moment to moment as the craft progresses on the selected course is that the wind, water, heading and sail setting variables are all generally interrelated and subject to independent variation throughout the voyage. On the other hand, the quantitative indication of hull speed by means of some sensor on the hull that is in contact with the water necessarily involves a time lag to indicate the result of a given adjustment in any of the variables available to the helmsman for adjustment such as sail setting since the increment of force resulting from a change in setting requires a certain amount of time to alter the momentum and thus the speed of the sailboat. Where relative wind speed indications are used as a guide, these readings, of course, are directly influenced by changes in both the wind speed and wind direction as well as the heading direction of the craft with respect to a constant wind velocity. There is thus in the prior art no rapidly responsive instrument for indicating factors directly related to the driving force which determines the forward speed of the boat and which is relatively independent of changes in wind and heading variables.